Use Soup for downloads and set timeouts

    Downloading via GIO uses gvfs internally. Unfortunately gvfs does not
    set or handle connection timeouts, meaning that bad connectivity can
    lead to a total hang.
    
    Fixing gvfs is not easy:
    https://mail.gnome.org/archives/gvfs-list/2013-July/msg00000.html
    
    And while looking at the bug, I realised just how much overkill this
    is. gvfs runs in a separate process and does the HTTP communication,
    communicating back to gio over dbus, and has a few layers of abstraction.
    
    The gio developers on IRC recommended just to use libsoup directly,
    which is not hard. And we can set timeouts appropriately in order to
    handle bad connectivity.